Constrained filament niobium-based superconductor composite and process of fabrication

ABSTRACT

A niobium-based superconductor is manufactured by establishing multiple niobium components in a billet of a ductile metal, working the composite billet through a series of reduction steps to form the niobium components into elongated elements, each niobium element having a thickness on the order of 1 to 25 microns, surrounding the billet prior to the last reduction step with a porous confining layer of an acid resistant metal, immersing the confined billet in an acid or a high temperature liquid metal to remove the ductile metal from between the niobium elements while the niobium elements remain confined by said porous layer, exposing the confined mass of niobium elements to a material capable of reacting with Nb to form a superconductor.

RELATED APPLICATIONS

[0001] This application is in part continuation of my patentapplications Ser. Nos. 09/532,362 filed Mar. 21, 2000, and 09/753,200filed Jan. 2, 2001, and 10/037,935 filed Jan. 2, 2002. The subjectmatter of said parent patent applications is included herein in itsentirety.

TECHNICAL FIELD

[0002] The present invention pertains to the fabrication of A-15 typemultifilament composite superconductors (By “A-15” is meant theintermetallic compounds having βW structure). These include Nb₃Sn, andNb₃Al and are important because of their superior high field properties.Unfortunately, they are brittle compounds, difficult to make as finefilaments and as a result are very expensive. For these reasons, ductileNbTi superconductors have dominated the commercial market even thoughtheir maximum magnetic field are limited to less than 8 Tesla.Significant improvements are needed in order to commercialize the A-15conductors and to extend the useful magnetic fields to the 12 Teslarange on a cost effective basis. The present invention is alsoapplicable to the production of the “B1” superconductors NbN and NbC.

STANDARD INDUSTRIAL PRACTICE

[0003] A detailed description of present day methods currently beingused in the industry is described in an article entitled “A-15Superconductors” in the Metals Handbook, Tenth 1 Edition, Volume 2 onpages 1060-1077, authored by David B. Smathers. Two processes arecurrently being used. One employs a bronze alloy as the matrix, theother a combination of pure copper and a pure Sn core. The first isknown as the “Bronze Process” and the latter the “Internal Tin Process”.The bronze matrix contains up to 13 wt % Sn, work hardens rapidly andrequires frequent annealing steps. These anneals are avoided with theinternal tin process. However, the cold drawing in the Internal Tinprocess can result in poor bonding, degraded filament quality and pooryields. After final drawing and twisting, both types of conductors areheated to approximately 700° C. for 200 hours or more to form Nb₃Sn.Magnets are made exclusively by the Wind and React method.

[0004] The current densities that are obtained are substantially belowwhat is theoretically possible based on experimental short sample data.Contributing to this problem is the low reaction temp of up to 700° C.,requiring hundreds of hours of reaction heat treatment. As the Sn isdepleted, the Sn gradient is reduced which further limits the reaction.Unreacted Nb in the Nb filament can be left and Kirkendall type voidsare formed in the residual matrix resulting in a lowering of the currentdensity and mechanical properties of the conductor.

[0005] Significant improvements are needed to improve the high fieldperformance and to reduce the cost of these important A-15 conductors.

BACKGROUND ART

[0006] In the fabrication of Nb₃Sn superconducting wire, a barrier,usually tantalum or a tantalum alloy is employed to prevent tincontamination of the stabilizing copper on the exterior of the wireduring heat treatment. The process is described in the article by DavidB. Smathers. While the porous metal sheath described in the presentinvention is similar to the barrier used in Nb₃Sn conductor fabrication,application of the technology in this invention is entirely unique.

FIBER PRODUCTION

[0007] In my earlier U.S. Pat. Nos. 5,034,857 and 5,869,196, I disclosea novel approach to the production of very fine valve metal filaments,preferable tantalum, for capacitor use. The benefits of fine filamentsrelative to fine powders are higher purity, lower cost, uniformity ofcross section, and ease of dielectric infiltration, while stillmaintaining high surface area for anodization. The uniformity of crosssection results in capacitors with high specific capacitance, lower ESRand ESL, and less sensitivity to forming voltage and sinteringtemperature as compared to fine powder compacts. Other patents involvingvalve metal filaments and fibers, their fabrication, or articles madetherefrom include U.S. Pat. Nos. 3,277,564 (Webber), 3,379,000 (Webber),3,394,213 (Roberts), 3,567,407 (Yoblin), 3,698,863 (Roberts) 3,742,369(Douglass), 4,502,884 (Fife), 5,306,462 (Fife) and 5,245,514 (Fife).

PRIOR ART

[0008] The prior art relating to the fabrication of A-15 conductors canbe obtained by reading both Smather's article and “Filamentary A-15Superconductors” by Masaki Suenaga and Alan F. Clark, Plenum Press, N.Y.Cryogenic Material Series (published 1980). In this book, the article byC. H. Rosner, B. A. Zeitlin, R. C X. Schwall, M. S. Walker and G. M.Ozeryansky entitled “Review of Superconducting Activities at IGC on A-15Conductors” pages 67-79, specifically summarizes the earlierdevelopments. Initially, powder metallurgy methods were employedfollowed soon by surface diffusion of liquid Sn of both Nb tapes andwires. Allen U.S. Pat. No. 3,218,693 describes a method where Sn coatedNb ribbons and wire were reacted to form Nb₃Sn at temperatures between800° C. to 1000° C. Similar products were also made by General Electricand later by IGC. The Nb wires in cable form, were Sn dipped, wound intoa magnet and reacted; D. F. Martin et al U.S. Pat. No. 3,429,032. Asubsequent article, by Scanlan and Fietz, “MultiFilamentary Nb₃Sn forSuperconducting Generator application”, IEEE Trans. MAG-11 page 287,March '75, describes fabrication of a Nb₃Sn cable employingelectroplated Sn as the Sn source.

BRIEF SUMMARY OF THE INVENTION

[0009] A new approach is necessary to improve the processing of Nb₃Snconductors. In the early 1960's, pure Nb tapes, wires and cable weredipped in molten Sn baths which was then reacted at high temperatures toform Nb₃Sn. Because Nb₃Sn is brittle, a ductile substrate of unreactedNb was left to permit handling and subsequent winding into magnets.However, the need for stable, fine filaments and twisted conductors soonmade this method obsolete.

[0010] My earlier U.S. Pat. No. 5,896,196 describes a process used tomanufacture Ta capacitors where Ta multifilaments are made in aconstraining sheath. This process describes the removal of the coppermatrix after the final forming operation. The advantages of the externalsheath is that final packaging of the filaments are unnecessary sincethe filaments are now constrained and supported by the outer sheath.Furthermore, the area inside the sheath is exactly determined as is thevolume fraction of Ta.

[0011] A precursor wire, containing fine Nb filaments enclosed in aconstraining sheath which can act as a supporting structure is produced.Upon the removal of the copper matrix, and employing a liquid Sn dippingprocess, Sn or a CuSn alloy is used to infiltrate and surround the Nbfilaments. The significant advantage here is that the need forsubsequent wire drawing is completely eliminated as is the co-processingand low yield difficulties of present day Nb₃Sn conductors. The abilityto easily increase the Sn concentration can result in substantialimprovement in current density over present day conductors. The sheath,in the preferred embodiments, is made of Nb although Ta and stainlesssteel could also be used. The sheath is fabricated by methods asdescribed in my aforesaid U.S. Pat. No. 5,869,196. The billet isprocessed in the normal manner by extrusion and wire drawing to thefinal size. The copper matrix is then removed from this precursor wireand replaced with a Sn or CuSn alloy matrix. Final reaction heattreatments are then used to convert the Nb to Nb₃Sn.

[0012] In an alternative embodiment, the original copper matrix isremoved from the confined billet by immersing the confined billet in amolten bath of a displacing metal at elevated temperature. The metal,which should be an alloying metal with copper, preferably comprises tin,aluminum, or an alloy of tin or aluminum, and should be heated to atemperature in the range of about 700 to 1200° C., preferably about 800to 1100° C., more preferably about 950° C. The high temperature metalbath essentially instantaneously dissolves the copper matrix andreplaces the copper matrix with itself. In a preferred embodiment, ahigh temperature copper-tin alloy bath is employed in place of the puretin, and replaces the copper with a copper/tin alloy.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a schematic depiction of the primary billet used in thepresent invention, FIG. 1a showing a transverse cross section, and FIG.1b showing a cutaway view revealing the longitudinal disposition of thebillet components.

[0014]FIG. 2 is a schematic depiction of the transverse cross section ofthe secondary billet used in the process of the present invention.

[0015]FIG. 3 is a schematic depiction of the product of the preferredembodiment of the present invention, FIG. 3a showing the product as acylindrical body, and FIG. 3b showing the product after shaping into arectangular body.

[0016]FIG. 4 is similar to FIG. 3 but shows the internal copper core toachieve stabilization.

[0017] FIGS. 5-9 show additional systems for constraining the niobiumfilaments during and after the step of acid leaching of the copper.

[0018]FIG. 10 is a schematic depiction of the primary billet used in thepreferred embodiment of the present invention.

[0019]FIG. 11 shows an over lapping design of confining layer.

[0020]FIG. 12 is a Cu—Sn phase diagram.

[0021]FIG. 13 shows the relationship between magnetic field and criticalcurrent for a number of different superconducting alloys. This is takenfrom U.S. Pat. No. 4,224,087.

[0022]FIG. 14 shows the thickness of the Nb₃ Sn layer as a function ofHf additions to the base Nb. This is also taken from U.S. Pat. No.4,224,087.

[0023]FIGS. 15a and 15 b are block flow diagrams depicting analternative fabrication method in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0024] This invention is directed to the fabrication of the fine metalfilaments for use as the base for reaction with tin (for example) toproduce A-15 superconductors in situ. A very small body of constrainedfilaments and a method for manufacture are disclosed. The invention willbe of greatest value in the superconductor industry, which seeks todevelop increasingly high magnetic field A-15 superconductors. The metalis selected from among the metallic elements, niobium and in particular,the A-15 compounds of Nb. In a preferred form, the product of thepresent invention, the metal filaments are of a diameter less than 50microns. They are constrained within a cylindrical sheath, also made ofmetal, which has a wall thickness of 100 microns or less. The metal thatforms the sheath is preferably the same as that of the filaments, but itis not necessarily so; tantalum can be used as well as stainless steel.Multifilament conductors suitable for use as superconductors are formedthrough the reduction of a metal billet consisting of multiple filamentsof an appropriate refractory metal, preferably niobium, containedwithin, and spaced apart by, a ductile metal, preferably copper. The Nbfilaments are elongated and substantially parallel within the billet asshown in FIG. 2. The array of Nb filaments 4 within the billet issurrounded by a confining layer of metal 7. This metal 7 is preferably,but not necessarily, the same as that which forms the filaments. Themetal layer 7 preferably completely surrounds the Nb filament array 4circumferentially and runs the full length of the filaments. The layeris separated from the filament array 4 by he same ductile metal 6 (e.g.Cu) that serves to separate the filaments 4 from each other. This sameductile metal 6 can also form the surface of the billet, preventingexposure of the confining metal layer during heating etc. The billet isreduced by conventional means, such as extrusion and wire drawing. Theconfining metal sheath 7 is then perforated (as shown in FIGS. 3a and 3b at 8) such that, in the case of one embodiment of the invention, themineral acid can readily diffuse through the porous layer and remove theductile copper separating the Nb filaments. It is important that theseperforations 8 should be designed such that the sheath retains itsability to restrain and support the loose Nb filaments 4. Moreimportantly, they should not substantially weaken the strength orductility of the entire composite. Preferably, the perforations 8 can beaccomplished by mechanically rolling the wire in a rolling mill with thedesired perforation pattern in a continuous matter. Other means can bealso employed, such as selective chemical etching, laser drilling, etc.as would be apparent to one skilled in the art. The constraining metalmust be one that is inert to the acid used to dissolve the copper.

[0025] Typical shapes would be a circular FIG. 3a or rectangular crosssection FIG. 3b. An advantage for the rectangular shape is that thedistance for the acid to penetrate for complete copper removal isreduced. Internal copper stabilization can be introduced as shown inFIG. 4. The Ta clad copper appears as a central core with a Ta diffusionbarrier layer and it also reduces the leaching time. Thereafter, thecopper-free confined bundle of Nb filaments is infiltrated with liquidSn or CuSn alloy to thoroughly coat and embed each Nb filament in asolid Sn or CuSn alloy matrix. The Sn is then reacted with Nb to formNb₃Sn in a separate step. Temperatures between 700° C. to 1100° C. areused, both performed in an inert atmosphere or vacuum chamber. TheNb-filaments may be reacted partially, completely or it can be completedat a later stage in the manufacture, e.g. after cabling or winding in amagnet.

[0026] In one preferred form of the invention, the Nb filaments, at thecompletion of the mechanical reduction step, will have a diameter on theorder of one to twenty-five microns. In this preferred final form of thecomposite, the Nb filaments are separated by the Cu matrix and the Nb-Custructure is surrounded by a 50 micron or less thick layer of Nb havingan effective porosity of 50% or less. When this composite is immersed inan acid leaching bath of HNO₃ and H₂O at 100° C., the copper is removedin about 60 min, leaving a bundle of micron size Nb filaments confinedby a porous Nb sheath. When this copper-free mass of Nb filaments isimmersed in a Sn alloy bath between 700° C. to 1100° C. the surfacetension of the molten Sn bath and the capillarity of the bundle of Nbfilaments enclosed within the sheath draws the tin into the bundle whereit completely surrounds each Nb filament. The Sn is then reacted withthe Nb to form the Nb₃Sn compound at a later stage of manufacturing.

[0027] As will be apparent to one skilled in the field of metallurgy,this porosity can be achieved, for example, by mechanically perforatingthe confining layer wherein the perforations 8 are uniformly spacedabout 0.25 mm apart with a diamond shape size of 0.13 mm×0.25 mm, asshown in FIGS. 3a and 3 b.

[0028] When the acid leaching of the copper from the Nb-Cu matrixresults in unacceptably high amounts of surface contamination on the Nbfilaments, these contaminates can be removed by the techniques describedon page 9, lines 38-55, in my earlier U.S. Pat. No. 5,869,169. Suchdeoxidation treatments may improve the wetting of the Nb filaments bythe liquid Sn alloy as well.

[0029] Prior to copper removal, multiple strands of wire can be cabledtogether and compacted if required. This would avoid any mechanicaldamage to the Nb filaments because, in this condition, the wire with theCu matrix is in its most ductile state. The completed cable can then beleached and Sn infiltrated.

[0030] The conductor of Example I contains within the niobium sheath65.1% copper by volume. High percentage of copper, i.e., greater spacingbetween Nb filaments, would permit a more rapid rate of copper removalby chemical etching but would also lower the current density of theconductor. To maximize the current density, the etched conductor can bemechanically compacted to increase the overall volume fraction of Nbbefore infiltration.

[0031] The infiltration step is performed in a continuous fashionsimilar to that used for Sn dipped Nb tapes. The resident time in the Snbath should be as short as possible, only long enough to allow the wireto reach temperature and to permit the Sn to completely infiltrate andembed the filaments. This also avoids any early brittle Nb₃Sn formationand the potential damage that can result from further processing such ascabling, etc.

[0032] Approximately 2 wt % Nb is soluble in liquid Sn at 1000° C.Extended time in liquid Sn baths at these temps can result in some Nbloss. Because of this, the time for the infiltration should be veryshort. In addition, this can largely be avoided by saturating he Snprior to infiltration by addition of pure Nb metal to the bath. This canbe done for example by slowly running ribbons or wires of pure Nbthrough the process first. At temperatures below 950° C., othernon-superconducting compounds can form rapidly. For these reasons thedipping temperatures should be at least 1000° C. and above. It is alsoimportant that the wire be cooled rapidly after infiltration to avoidthe formation of large grain size brittle intermetallic compounds of Cuand Sn. Large copper additions to the Sn bath and its effects on Nb₃Snlayer growth has been reported by J. S. Caslaw, Cryogenic, February '71,pp. 51-59. The presence of Cu “catalyze” the Nb₃Sn reaction and improvesthe surface of the Nb₃Sn. Addition of up to 32 wt % Cu resulted insubstantial increase in J_(c).

[0033] The Hc2 and Tc values of Nb₃Sn are influenced by the Nb alloycomposition. It is well known that selective additions of Ti and Ta canincrease these values. It's important to not only increase Hc2 and Tcbut also the J_(c) properties through proper flux pinning mechanisms.Smathers and Swenaga's articles give detailed explanation of “Fluxpinning” and the methods currently employed. All of these methods can beapplied in this invention. Besides pure Nb, alloys of Nb-1 wt % Zr,Nb-1.5 wt % Ti, and Nb-7.5 wt % Ta can be used. The alloys can also bemechanically added to the Nb by wrapping alternating thin layers ofeither Ta, Ti, Zr, or Hf between the Nb layers in a Jelly Roll fashion.The Ta, Ti, Zr, or Hf can also be in the form of an open mesh design. Asingle thin layer can also be used on the surface of the Nb filament.This would help to reduce the possibility of bridging between filaments.The Sn bath would include, in addition to copper, smaller amounts of Ti,Mg, Al, Zr, and Ga as well. The Ta and Ti can also be added to the Nb bymeans of a thin surface layer of expanded Ta or Ti mesh. The Sn bathcould include in addition to Cu, smaller amounts of Mg and Ti.

[0034] The most important parameter determining the performance of Nb₃Snconductors is its current density. In the field of High Energy Physic,accelerator magnets capable of operating in excess of 12 Tesla at 4.2°K. are needed. The dipole magnets for the Large Hardon Collideraccelerator being constructed at CERN can only operate at less than 10Tesla at 1.9° K. using NbTi. For Nb₃Sn, the highest J_(c) values arecurrently being made by the Internal Sn process and values as high as2070 A/mm² at 12 Tesla and 4.2° K. have been reported. The bronzeprocess appears to be limited to below 1,000 A/mm² and is thus not afactor. Values as high as 3,000 A/mm² are needed for the next generationof accelerator magnets.

[0035] The area within the diffusion barrier contains only threecomponents; Nb, Cu and Sn. Increasing J_(c) can only be obtained byincrease in the volume fraction of Nb. This requires a proportionalreduction in copper which increases the Sn to Cu ratio. Copper simplyacts as a carrier for the Sn; it does not by itself participate directlytoward increasing J_(c). It does however perform a vital function; thatis to allow the successful co-processing of both the bronze and InternalSn Nb₃Sn conductors. Intensive efforts directed over the past twodecades toward optimizing the Nb:Cu:Sn ratios has all but reached itslimits due to the concomitant decrease in its fabricability as the Snfraction is increased. In the present invention, no such limitationexists. The matrix alloy can be varied between pure Sn to pure Cu.Fabricability is not an issue since the conductor is made initially witha pure copper matrix.

[0036] While the preferred method of application of the constrainingsheath has been described, it is recognized that alternatively, thesheath can be separately applied to a finished copper niobiummulti-filaments wire by mechanical means. Several additional techniquesmay be employed as shown in FIGS. 5 through 9.

[0037] In FIG. 5, the constraining layer is spirally wrapped around theCu—Nb composite with adequate spacing between the spirals to allowaccess by the leaching acid.

[0038] In FIG. 6, the constraining layer is woven in a braid around theCu—Nb composite with an open weave.

[0039] In FIGS. 7 and 9, the Cu—Nb composites are carried in grooves ofa carrier element resistant to the leach, but sufficiently confining toconstrain the Cu free Nb filaments.

[0040] In FIG. 8, a Cu—Nb cable is twisted and flattened and thenpartially surrounded by a constraining layer.

[0041] In all cases, the principal requirements of the constraininglayer are to allow for sufficient porosity for copper removal by acid,to be resistant to nitric acid attack, to constrain and support theloose Nb filaments after Cu removal and for subsequent infiltration withSn. The constraining metal must be one which will not react with Sn or,if Nb is used will be sufficiently thick so that it will not be allconverted to Nb₃Sn.

[0042] Transition metal carbides and nitrides such as NbN and NbC occurwith the B1 (NACL) crystal structure. Niobium nitride has been shown tohave a transition temperature approaching 19° K. These importantcompounds are described in “Treatise on Material Science andTechnology”, Vol. 14, Metallurgy of Superconducting Materials, 1979edited by Thomas Luhman and David Dew-Hughes, pages 429-432.

[0043] The present invention can be employed for forming NbN. Exactlythe same steps are utilized, that of porous outer sheath surrounding acopper matrix with Nb filaments from which the copper matrix is removed.Instead of the Sn infiltration step, a nitrogen containing atmosphere isintroduced in which the niobium filaments are converted into niobiumnitride (NbN) at elevated temperatures.

[0044] From a practical point of view, the gaseous phase reaction is asimpler and cleaner process than the handling of liquid Sn in thetemperature range of 1000° C. The NbN filaments can be used as reactedor may require a metal infiltration for improved stability reasons asdescribed by L. T. Summers, J. R. Miller, “The Influence of Liquid MetalInfiltration on Superconducting Characteristics of Niobium Nitride,”Advanced in Cryogenic Eng., Vol. 34, pp. 835-842, 1987.

EXAMPLE I

[0045] 19 holes 2 are drilled into a 15.3 cm diameter bar 1 of copper63.5 cm long 1 as shown in FIG. 1a. The holes are 2.57 cm in diameterand run the length of the bar in parallel fashion. The pattern of holesis as shown in FIG. 1a. The shortest distance, between any two holes is5.08 mm. A 1.27 cm insert is machined into each end of the copper bar.The insets are necessary in order that a copper nose and tail may beattached later. The copper bar containing the holes, a copper nose, anda copper tail are etched clean in a nitric acid solution, they arerinsed in water, rinsed a second time in methanol, and are then dried.19 niobium bars, 2.54 cm in diameter and 61 cm long 2 are wiped cleanwith acetone and are inserted into the holes in the copper bar 3. Thenose and tail are tungsten inert gas (TIG) welded into place, and thebillet is evacuated at a temperature of 427° C. to a pressure of 10⁻⁶torr. The billet is then sealed as shown in FIG. 1b.

[0046] In preparation for extrusion, the billet is heated at atemperature of 816° C. for a period of three hours. The billet is thenextruded to a diameter of 2.54 cm. The extruded rod is cropped to ensureuniformity, and the cropped rod is cold drawn at an a real reductionrate of 20% per die pass to a final hexagonal diameter of 3.48 mm. Thatis, the final wire shape is hexagonal and the distance from flat to flatacross the hexagon is 3.48 mm. At this size, the niobium filamentdiameter is 0.61 mm.

[0047] The wire is straightened and cut to 61 cm lengths. Pure copperrod is drawn to 3.48 mm diameter hexagonal wire and is straightened andcut to 61 cm lengths in the same manner as the wire containing the Nbfilaments. Both types of filaments are cleaned in nitric acid in thesame manner as was the primary billet. 1045 of the Nb-containingfilaments 4 are stacked in symmetrical fashion.

[0048] A section of 0.64 mm thick Nb sheets 7, 45.7 cm wide and 610 cmlong, is wiped clean with acetone and is inserted circumferentially intoa clean copper can 8 having an internal diameter 14.5 cm, an externaldiameter 16.5 cm, and a length of 63.5 cm 7. The sheet 7 overlays by 0.3cm so as to ensure a continuous layer. The long dimension of the sheetruns along the can length. The stack of filaments is inserted into theniobium-lined can (see FIG. 2) and the copper nose and tail are electronbeam welded into place under a vacuum of 10-4 torr. The billet is thenhot isostatically pressed at a pressure of 104 Mpa and temperature of650° C. for four hours. The isostatically pressed billet is machined toa diameter of 15.3 cm, and the billet is prepared for extrusion byheating for three hours at 816° C. The billet is then extruded to adiameter of 2.54 cm.

[0049] The extruded rod is cropped to ensure uniformity. It is thendrawn at an areal reduction rate of 20% per die pass to a diameter of1.02 mm and twisted. At this wire diameter, the diameter of the Nbfilaments is 4.06 microns, and the thickness of the niobium sheath is42.6 microns. The total volume fraction of Nb filaments within the Nbsheath is 34.9%.

[0050] Thereafter, the Nb outer sheath 7 is perforated by means such asslotted rolls as mentioned previously and the product is leached inHNO₃/H₂O at 100° C. for 60 min. to remove the copper separating the Nbfilaments. The copper-free mass of Nb filaments is then preferablyimmersed in a tin bath at 1100° C. to allow for maximum fluidity andsurface tension of the Sn bath. An inert atmosphere of Argon is used toavoid oxidation. Dipping resident time in the Sn bath should be as shortas possible (less than 10 sec) to minimize Nb₃Sn formation at this stageand maximize the ductility for further processing; i.e. cabling.

EXAMPLE II

[0051] In Example I, a continuous sheath, as shown in FIG. 2, wasemployed and at the final size the conductor was mechanically perforatedas shown in FIGS. 3a and 3 b to allow for the removal of the coppermatrix by acid leaching followed by liquid tin infiltration at hightemperatures. It was also shown that the sheath can be appliedseparately after the conductor has been fabricated by means of tapingFIG. 5, braiding FIG. 6, or by mechanically confining the conductor intoa support structure as shown in FIGS. 7, 8 and 9.

[0052] A simpler and considerably less expensive method has been foundwhich accomplishes the same objective. The sheath is constructed exactlyas in Example I except now a narrow opening is left which is parallel toand runs the length of the filaments shown in FIG. 10.

[0053] The size of the opening can be adjusted to permit rapid copperremoval of the matrix by acid leaching and still retain the sheath'sability to constrain and support the filaments. Following the finalreduction, twisting and copper removal, a product which is an exactduplicate of the taping example of FIG. 5 is remarkably produced.Several variations are possible in the sheath design. For example, morethan one opening can be used.

[0054] The twisting action causes the outer filaments to stretch morethan the inner strands and when the copper matrix is removed, the outerfilaments are under tension and act to compress and constrain the innerbundle of filaments. It has been observed that untwisted or lightlytwisted, the outer filaments will tend to exfoliate at the sheathopening.

[0055] This conductor was leached in a HNO₃/H₂O 50/50 acid solution at100° C. to completely remove the copper matrix. The following Cu—Snalloys were then used for infiltration experiments: Cu wt % Sn wt % 8713 67 33 50 50 25 75 20 80 10 90 0 100

[0056] Successful infiltration at 1000° C. was obtained for all alloyswith the exception of the 13 wt % Sn alloy. This alloy has a liquidustemperature of 1000° C. and would have required a temperature of atleast 1200° C. which was beyond the temperature capability of our smalllaboratory furnace.

[0057] The 33 wt % Sn alloy sample was subsequently heat treated at 675°C. for 24 and 48 hours. Metallographic examination of the cross sectionfor the 48 hour sample reveals a substantial Nb₃Sn reacted layer,averaging 4-6 micron in thickness both in the filaments and in thesurrounding sheath.

[0058] As shown in FIG. 4, a central copper core can be used. As atypical requirement, up to 40 Vol % copper is needed to insure stableconductor performance. To prevent Sn contamination of the copper core, aTa protective barrier is employed in FIG. 4. It should be recognizedthat this structure by itself contributes a significant amount ofsupport and that together with the external sheath of this inventionwould combine to protect the fragile Nb₃Sn filaments inside. It has beenshown that a high strength copper containing Nb composite has beensuccessfully used in conjunction with Nb₃Sn conductors and can be usedin place of pure copper. (See Advances in Cryogenic Eng., Vol. 42,Plenum Press, NY 1996, pages 1423-1432).

EXAMPLE III

[0059] The open sheath design shown in FIG. 10, while improving the acidleaching of the Copper matrix can leave an open gap on the surface ofthe wire after dipping. In this condition, on bending especially in thetwisted condition, breaks can occur. An overlapping design, shown inFIG. 11 essentially overcomes this difficulty. The overlap designprovides a smooth continuous sheath on the surface of the wire and inaddition provides added support at the opening. When sufficientthickness and depth of the openings are provided, little differences inthe rate of copper removal were observed. In Example III, an opening of0.05 mm×1.00 mm was used. Furthermore, this design insures that all thefilaments are completely enclosed within the sheath and eliminates anypossibility of filament escape.

[0060] Test samples were drawn to 1.00 mm and 0.762 mm diameter. Thecopper matrix was removed and dipped in a Cu-33 wt % Sn bath at 1100° C.The total resident time in the CuSn bath was 10 seconds. It was thenreacted for 50 hours @690° C. in an Argon atmosphere furnace. Thesamples were then tested at 4.2° K. in a perpendicular magnetic field of9 and 8 Tesla: Sample No. Size 9T 8T 5006-4B 1.00 mm >120^(a) >120^(a)5006-4B 0.762 mm   102^(a)   108^(a)

[0061] The J_(C) calculation based on the original Nb area prior toreaction equals 1666 A/mm² @9 Tesla.

[0062] The ductility of samples processed with increasing Snconcentration were examined. While the higher wt % Sn sample gaveexcellent Nb₃ Sn thickness values, they were also brittle and this couldcompromise further processing of the wire. It's important to point outthat this brittleness is not caused by the Nb filaments but is entirelydue to the brittle CuSn compound.

[0063] Indirect evidence of matrix ductility was observed during themachining of the CuSn alloys from the used crucible. Normal proceduresallow the CuSn to furnace cool. It was found that all the high Sn alloyswere brittle, with the sole exception of the Cu-23 to 25 wt % Sn alloy.Continuous ductile chips were obtained. The reason can be seen from theCuSn phase diagram. Below 350° C., a significant amount (˜40%) ofductile a phase compound is formed and is in equilibrium with the εcompound phase.

[0064] A series of water quench experiments were carried out with theCu-23, 25, 27, 33 and 38 wt % Sn samples. These samples were quicklyremoved from the dipping furnace and water quenched. Bend test showedbrittle behavior for all the Sn alloys except for the 23 to 25 wt % Snalloy. It could, remarkably be bent around a ¼ diameter mandrel withoutfracture. For this reason, Cu-23 to 25 wt % Sn based alloy was selectedfor most of the matrix alloy studies.

[0065] To increase the Nb filament packing, the leached precursor wire,which measured 0.914 mm diameter, was further twisted and compacted to adiameter of 0.813 mm. The overall reduction in area was 20.9% and thusresulted in a proportional increase in the overall non-copper currentdensity of the wire. Additionally, as the wire is drawn through themolten metal bath, the twisted sheath tightens and further compressesand constrains the filament bundle.

EXAMPLE IV

[0066] In this invention, the ability to fully utilize alloyingadditions both to the Nb filaments and to the matrix without themechanical co-processing limitations of the existing processes offerunique opportunities to further enhance the performance of these brittleA-15 compound conductors. In addition to the references cited earlier,more recent references are included and summarize the results of threedecades of continuous effort, which is still ongoing today. Of specificinterest are: Tachikawa, “Filamentary A-15 Superconductors” by MasakiSuenaga and Alan F. Clark, Plenum Press, N.Y., Cryogenic Mat. Series1980 Page 1-12. Tachikawa, et al U.S. Pat. No. 4,224,087, Flukiger, etal U.S. Pat. No. 4,746,581, and Sakamoto, et al “Very High CriticalCurrent Density of Bronze-processed (Nb, Ti)3 Sn Superconducting Wire.”IEEE Transaction, ASC volume 10, No. 1, March, 2000.

[0067] The following Nb alloys were studied; Nb-1 wt % Zr, Nb-1.8 wt %Ti, Nb-7.5 wt % Ta, and Nb-10 wt % Hf-1 wt % Ti (C-103 Wah Chang). It isimportant to note that all of these are commercially available; theNb-1.8 wt % Ti and Nb-7.5 wt % Ta having been specifically developed forthe superconductivity market. The Nb—Hf alloy however was not, havingbeen developed almost four decades ago for aerospace applications. Itwould be extremely fortuitous, of great practical and commercial value,if alloy C-103 can be used in this invention, especially considering thetime and expense normally required for such a task.

[0068] Of particular relevance is Tachikawa '087 where in FIG. 7 andTable 1, (our FIG. 12 and FIG. 13) clearly shows current increases at 12Tesla of over 4 times and Nb₃Sn thickness layers of more than 2 times inthe Hf alloyed Nb as compared to pure Nb. In addition, in hispublication, he reported that no degradation occurs until the mechanicalstrain exceeds 1% as compared to 0.7 without Hf additions.

[0069] Martin, et al '032 and more recently, Rumaner and Benz, “Effectsof Oxygen and Zirconium on the Growth and Superconducting Properties ofNb₃Sn”. Met. and Mat., Trans. A, Vol. 25A, Jan. '94, P. 203-212describes the flux pinning role of oxygen and zirconium in the Nb-1 wt %Zr alloy. In this invention, the same mechanism can be employed. In myaforesaid U.S. Pat. No. 5,869,196, I describe the need to deoxidize thefine Ta filaments intended for capacitor applications. The filamentsizes of interest were 1 micron and less. As the filaments are reducedin size the surface area of the filament increases as the reciprocal(I/D) of the diameter. This would suggest that when smaller filamentsare used, the surface area and thus also the oxygen on the surface ofthe filament would also increase. During the leaching step, a strong50/50 HNO₃/H₂O solution, heated to 100° C. is used. Substantial amountof surface oxygen can be generated. This oxygen can then be combinedwith zirconium as described by Martin and Rumaner and Benz for fluxpinning.

[0070] The benefits of using Nb alloys such as Nb 1 wt % Zr, Nb 1.8 wt %Ti, Nb 7.5 wt % Ta, and

[0071] Nb-10Hf-1 wt % Ti were confirmed in our studies. Substantialincrease in Nb₃Sn layer thickness compared to pure Nb were seen in allcases. The increases for Nb-10Hf-1 wt % Ti was by far the greatest,confirming the results of Tachikawa. The following samples were dippedin a Cu-23 wt % Sn bath at 1100° C. and reacted for 100 hours at 690° C.Nb Alloy Nb₃Sn Thickness (Microns) Nb 2-4 Nb-1 wt % Zr 6-8 Nb-1.8 wt %Ti 6-8 Nb-7.5 wt % Ta 4-6 Nb-10 wt % Hf-1 wt % Ti 10-12

[0072] Nb 1 wt % Zr appears to react as well as Nb 1.8 wt % Ti. Thisshould not be surprising since Ti and Zr are very similar metals andwould be expected to function in a similar manner.

[0073] The CuSn matrix alloys studied were: Cu-19 Sn-0.37 Ti, Cu-23Sn-0.25 to 4 wt % Ti, and

[0074] Cu-23 Sn-0.5 to 2 wt % Mg. It was observed that any increase ofover 2 wt % Ti and Mg resulted in incomplete infiltration. It wassurprising that this would occur at this level of alloy addition andmust be related to changes in the surface tension of the bath and itslimited solubility, even at 1100° C. Further, it was found that alloyswith less than 0.5 wt % Ti were equally effective in promoting Nb₃Sngrowth. This should be expected since the Nb₃Sn is reacted is at 700°C.±50° C. and the solubility limits for Ti and Mg are lower at lowertemperatures. For these reasons, the preferred alloy matrix compositionshould be between 15 to 25 wt % Sn plus 0.2 to 2.0 wt % Ti, balancecopper. A significant discovery, which was not anticipated, was theapparent complete absence of Kirkendall voids from all the reactedsamples. This is in stark contrast to similarly reacted samples ofcommercial internal Sn conductors as described in a paper by M. Naus, etal, “The Interdiffusion of Cu and Sn in Internal Sn Nb₃ SnSuperconductors”, IEEE Transaction ASC Volume 10(1) 983-987, 2000. Themajor source of Kirkendall voids, which occurs even before the Nb₃Snreaction starts, is the direct result of the Cu and Sn diffusionprocess.

[0075] Because of the absences of these defects, it is expected that theproduct of this invention would be superior mechanically compared tointernal Sn conductors and would be more resistant to degradation, fromcabling, compacting and other externally applied stresses.

[0076] EXAMPLE V

[0077] The previous examples describe removal of the original coppermatrix by acid leaching followed by a high temperature liquid Cu/Snimpregnation step. This example provides an alternate method for removalof the original copper matrix.

[0078] This example starts with the premise that it would not benecessary to remove the original copper matrix if a satisfactory way canbe found to add Sn to the copper; typically between the 13% and 45 wt %range. Assurance of complete copper removal is difficult and timeconsuming. Moreover, hot acid removal can result in surface oxygen andnitrogen contamination as well as left over residue copper nitrates, allof which can reduce wetting and successful impregnation.

[0079] It has been observed that when a copper rod is inserted into anSn bath at 1100° C., the copper rod dissolved essentially instantly.Based on this observation, it was postulated that this essentiallyinstantaneous dissolving could occur also with the copper matrix and ineffect replace the copper with a Sn matrix. To limit the % Sn to thedesired 13-45 wt % range, a Cu—Sn alloy could be used in place of pureSn. Increasing copper would be expected to slow the alloying process soit was important to establish the limits of both bath composition andtime. Referring to FIG. 15a, in accordance with this Example V, samplesof billets taken in the as drawn unetched condition 100 were immersed ina pure Sn bath 102 at 1100° C. for 2, 4, 6, and 8 minutes. The wirediameter was 1 mm with the billet containing 2,5088 micron Nb filaments.

[0080] Complete Sn penetration was observed even for the 2-minutesample. Duplicate samples were held in a Cu-25 wt % Sn bath for 5, 10and 15 minutes. Limited Sn diffusion was observed even for the 15 minutesamples. The Sn was limited to the open gap region of the sheath. Theextend of the Sn diffusion can be seen by the Nb₃Sn that forms on eachfilament when Sn is present in sufficient quantities. Higher temperatureimproves Sn diffusion simply by increasing the liquidity and mixingbetween the Cu—Sn bath and copper matrix.

[0081] Referring to FIG. 15b, in a preferred embodiment of thisalternative process of this invention, a two-step dipping process isused: samples of billets 100 are subjected to a first dip 102 in a pureSn bath at 1100° C. followed by a dip in a Cu-25 wt % Sn bath 104 at1100° C. In both cases, the high liquidity of the Sn is utilized toreplace the copper matrix to Sn and for the reintroduction of a fixedquantity of Cu back into the matrix.

[0082] Previous samples of the pure Sn dip for 2, 4, 6 and 8 minuteswere redipped a second time for 2, 4, 6 and 8 minutes. Sample Pure SnCu-25 Sn 1 2 2 2 4 4 3 6 6 4 8 8

[0083] Successful impregnation was observed in all cases.

[0084] In a surprising discovery, using this two step process, theoriginal filament pattern was essentially retained. In the case wherethe copper matrix was acid removed and impregnated with a CuSn alloy,the filaments appear to agglomerate in a random fashion. The reason forthis retention is probably due to the initial formation of a CuSn twophase liquid/semi-solid state which would tend to hold the filaments onplace.

[0085] Ti, Mg and other alloying elements also may be included in secondbath. The advantage of adding Ti, Mg and other alloying elements aredetailed in Example IV. The difficulties of adding these elements arenow eliminated since during the first dip in a pure tin bath, maximumliquidity are still maintained.

EXAMPLE VI

[0086] In Examples V, it was shown that this acid leaching step could bereplaced with a two step dipping operation; first, with a pure Snfollowed by a Cu Sn Alloy dip. An important observation was made duringthis experiment. It was found that the wire shape and more importantlythe original spacing and filament pattern was unchanged. This occurreddespite the fact that Sn at this temp should have instantly dissolvedthe copper matrix and allowed all the filaments to move freely. Thisunexpected behavior suggested that an external sheath may not benecessary and if so would greatly simplify the entire process.

[0087] The following experiment was made with a bronze 13% Sn matrix Nbfilaments wire with a central copper core protected by a Ta barrier. Thewire specification were as follows:

[0088] Diameter=0.78 mm

[0089] Matrix=Copper 13% Sn

[0090] Nb Filaments=4 microns

[0091] No. of Filaments=6,000

[0092] Bronze/Nb Ratio=2.21

[0093] Area % Cu/Ta core=23.6%

[0094] The wires were tested in the twisted and untwisted conditions asshown below. Pure Sn Cu25 Sn Sample Treatment Temperature TimeTemperature Time 4016-6 Untwisted 1000° C. 1 minute 1050° C. 1 minuteCable 4016-6 Twisted 1000° C. 1 minute 1050° C. 1 minute Cable

[0095] In the case of the untwisted wire, the bronze matrix wascompletely dissolved leaving a mass of loose tangled filaments while thetwisted sample remained essentially unchanged. Only the thin outercontinuous bronze layer was removed and the wire measured 0.74 mm indiameter. Metallographic examination confirmed that the originalfilament/matrix pattern was basically unchanged. Using copper ratherthan a bronze matrix should not change the results since the bronze 13%Sn has an even lower melting temperature than copper.

[0096] Twisting the wire is essential. As described previously, thetwisting action stretches the outer filaments to a greater extent thanthe inner strands. In so doing, it acts to constrain the inner filamentsmuch like an external sheath. This effect is shown in FIG. 5 and FIG. 6,where a thin ribbons or wires were taped or braided on to a compositewire. In the example described, the 0.78 mm diameter wire was twisted ata rate of 2 turns per cm. A higher twist rate, while desirable, canhowever reduce the porosity of the wire and possible extend the timenecessary for adequate Sn diffusion.

[0097] It is postulated that the retention of the filament pattern isdue to the presence of a two phase solid—semi solid Cu/Sn matrix. Thisrequires that some copper Sn Alloy remains after dipping and thatcomplete copper replacement by Sn is not required. To accomplish this,dipping temp should be kept below the melting point of pure copper 1083°C., and the spacing between filaments kept narrow to limit the copperloss.

[0098] In example V, a double-dip procedure, first a pure Sn followed bya Cu25Sn alloy dip was used. It has been found that the critical currentvalues obtained are better than for a single dip procedure such asdescribed in example VI. In theory, both procedures should be capable ofreaching the same level of performance, and to do so would requiremodification in both time and temperature used in the dipping process.

[0099] Significant increases in performance were also obtained as the Sncontent was increased in the second bath. This is shown below: SampleSize % Sn Ic @ 9T 26B/16A-0 0.5 mm 25 240 26B/16A-0 0.5 mm 38 35533B/30A 0.5 mm 45 500 33B/30A 0.5 mm 100 680

[0100] These results were confirmed by metallographic examination whichshowed that significantly more Nb was reacted with the higher Sn alloys.

[0101] A final reaction treatment is required, typically 50-150 hours attemperature between 600° and 750° C. much like that used for theinternal Sn conductors commercial available today. This is needed inorder to homogenize any Sn gradients that exists between the surface andthe center of the wire and to fully react all the Nb filaments.

[0102] The examples given above are illustrative in nature only andvariation in composition, temperature and time should be recognized andincluded in the scope of this invention.

What is claimed:
 1. A process for manufacturing a Nb₃Sn superconductorcomprising the steps of establishing multiple niobium components in abillet of a ductile metal, working the composite billet through a seriesof reduction steps to form said niobium components into elongatedelements, each said element having a thickness on the order of 1 to 25microns, surrounding said billet with a porous confining layer of anacid resistant metal, immersing said confined billet in an acid toremove said ductile metal from between the niobium elements while theniobium elements remain confined by said porous layer, immersing theconfined mass of niobium elements in a liquid metal containing tin tocoat said niobium elements with said liquid metal and subsequentlyreacting said tin with the Nb filaments to form NB₃Sn.
 2. The process ofclaim 1 wherein said reaction is accomplished after said filaments areformed into a magnetic coil.
 3. The process of claim 1 wherein theliquid metal bath is pure tin.
 4. The process of claim 1 wherein theliquid metal bath is a copper alloy containing up to 95 wt % Sn.
 5. Theprocess of claim 1 wherein the liquid metal bath is a copper alloycontaining up to 95 wt % Sn and at least 13 wt % Sn.
 6. A process formanufacturing a niobium-based superconductor selected from the groupconsisting of Nb₃Sn, Nb₃Al, NbN, and NbC comprising the steps ofestablishing multiple niobium components in a billet of a ductile metal,working the composite billet through a series of reduction steps to formsaid niobium components into elongated elements, each said elementhaving a thickness on the order of 1 to 25 microns, surrounding saidbillet with a porous confining layer of an acid resistant metal,immersing said confined billet in an acid to remove said ductile metalfrom between the niobium elements while the niobium elements remainconfined by said porous layer, exposing the confined mass of niobiumelements to a material capable of reacting with Nb to form asuperconductor.
 7. The process of claim 6 wherein said material isnitrogen.
 8. A process for manufacturing a A-15 or B1 type compoundsuperconductor comprising the steps of establishing multiple primarymetal component in a billet of a ductile metal, working the compositebillet through a series of reduction steps to form said primary metalcomponents into elongated elements, each said element having a thicknessin the order of 1 to 25 microns, surrounding said elements with a porousconfining layer of an acid resistant metal, immersing said confinedelements in an acid to remove said ductile metal from between theprimary metal elements while the primary elements remain confined bysaid porous layer, contacting the confined mass of primary metalelements with a reactant and reacting the primary metal elements withthe reactant to form the A-15 or B1 superconducting compound.
 9. Theprocess of claim 1 wherein said billet is surrounded by said porouslayer prior to the last reduction step.
 10. The process of claim 1wherein said porous layer is created by twisting the reduced billet sothat outer elements in the billet serve to confine inner elements in thebillet when the ductile metal is removed from the billet.
 11. Theprocess of claim 1 wherein said porous layer is created by running atleast one outer element along the outer surface of the reduced billetand twisting the reduced billet so that outer elements in the billetserve to confine inner elements in the billet when the ductile metal isremoved from the billet.
 12. A process for manufacturing a Nb₃Snsuperconductor comprising the steps of establishing multiple niobiumcomponents in a billet of a ductile metal, working the composite billetthrough a series of reduction steps to form said niobium components intoelongated elements, each said element having a thickness on the order of1 to 25 microns, surrounding said billet with a porous confining layerof Nb metal, immersing said confined billet in a displacing metal forsaid ductile metal at an elevated temperature whereby said displacingmetal replaces said ductile metal with itself to remove said ductilemetal from between the niobium elements while the niobium elementsremain confined by said porous layer, immersing the confined mass ofniobium elements in a liquid metal containing tin to coat said niobiumelements with said liquid metal and subsequently reacting said tin withthe Nb filaments to form Nb₃Sn.
 13. A process for manufacturing a Nb₃Snsuperconductor comprising the steps of establishing multiple niobiumcomponents in a billet of a ductile metal, working the composite billetthrough a series of reduction steps to form said niobium components intoelongated elements, each said element having a thickness on the order of1 to 25 microns, surrounding said billet with a porous confining layerof a solvent resistant metal, immersing said confined billet in adisplacing metal for said ductile metal at an elevated temperaturewhereby said displacing metal replaces said ductile metal with itself toremove said ductile metal from between the niobium elements while theniobium elements remain confined by said porous layer, immersing theconfined mass of niobium elements in a liquid metal containing tin tocoat said niobium elements with said liquid metal and subsequentlyreacting said tin with the Nb filaments to form Nb₃Sn, the liquid metalbeing a copper alloy containing up to 95 wt % Sn and at least 13 wt %Sn.
 14. The process of claim 13, wherein the metal bath contains about23 to 25 wt % Sn.
 15. The process of claim 13, wherein the liquid metalcoated niobium is rapidly quenched after removal from the metal bath.16. The process of claim 13, wherein the liquid metal bath contains 0.2to 2.0 wt % Ti.
 17. The process of claim 13, wherein the Nb componentcontains at least one metal from the group consisting of Zr, Ti, Ta andHf.
 18. The process of claim 17, wherein the Nb component is Nb-1 wt %Zr.
 19. The process of claim 17, wherein the Nb component is Nb-1.8 wt %Ti.
 20. The process of claim 17, wherein the Nb component is Nb-10 wt %Hf-1 wt % Ti.
 21. The process of claim 17, wherein the Nb component isNb-7.5 wt % Ta.
 22. A process for manufacturing a Nb₃Sn superconductorcomprising the steps of establishing multiple niobium components in abillet of a ductile metal, working the composite billet through a seriesof reduction steps to form said niobium components into elongatedelements, each said element having a thickness on the order of 1 to 25microns, surrounding said billet with a porous confining layer,immersing said confined billet in a liquid metal bath comprising a metalor metal alloy capable of alloying with the ductile metal whereby toreplace said ductile metal from between the said niobium elements withan alloy of said ductile metal and said alloying metal, reacting saidtin or tin alloy with the Nb filaments to form Nb₃Sn.
 23. The process ofclaim 22, wherein the liquid metal bath comprises tin, aluminum or analloy of tin or aluminum.
 24. The process of claim 22, wherein theliquid metal bath comprises a tin-copper alloy.
 25. The process of claim24, wherein the liquid metal bath comprises about 23 to 25 wt % tin. 26.The process of claim 22, wherein the liquid metal bath comprises puretin.
 27. The process of claim 22, wherein the liquid metal bath alsocontains at least one metal selected from the group consisting of Zr,Ti, Ta, Mg and Hf.
 28. The process of claim 27, wherein the liquid metalbath contains 1 wt % Zr.
 29. The process of claim 27, wherein the liquidmetal bath contains 1.8 wt % Ti.
 30. The process of claim 27, whereinthe liquid metal bath contains 10 wt % Hf and 1 wt % Ti.
 31. The processof claim 27, wherein the liquid metal bath contains 7.5 wt % Ta.
 32. Theprocess of claim 22, wherein the liquid metal bath is heated to atemperature of 700 to 1200° C.
 33. The process of claim 32, wherein theliquid metal bath is heated to a temperature of 1000 to 1100° C.
 34. Theprocess of claim 32, wherein the liquid metal bath is heated to atemperature of about 950° C.